1. Field of the Invention
The present invention relates to a communication system which performs signal multiplexing in use of a Code Division Multiplexing (CDM) method.
2. Description of Related Art
Conventionally, as a technique for multiplexing signal arrays of a plurality of channels, a Code Division Multiplexing method is known. As the Code Division Multiplexing method, such as Synchronous Code Division Multiplexing method, Asynchronous Code Division Multiplexing method and Quasi-Synchronous Code Division Multiplexing method are known. The Synchronous Code Division Multiplexing method is superior to other methods in that the number of multiplexing in a code length (that is, the number or chips) becomes larger. As a document disclosing the Synchronous Code Division Multiplexing method, Japanese Patent Application Laid-Open No. 2004-80385 is known, for example.
FIG. 9 is a schematic diagram showing an example of a communication system in which the Synchronous Code Division Multiplexing method is employed. In the example shown in FIG. 9, a transceiver 910 is connected to a plurality of receivers 930-1 to 930-n via a distributor 920. The transceiver 910 performs code division multiplexing for a plurality of signal arrays to transmit to the receivers 930-1 to 930-n. Each of the receivers 930-1 to 930-n demultiplexes a signal array corresponding to a relevant channel (one of channels CH1 to CHn) among the code division multiplexed signal arrays.
FIG. 10 is a block diagram showing an example of an internal structure of the transceiver 910. According to the transceiver 910 in FIG. 10, each of data generators 911-1 to 911-n generates a signal array to provide to the receivers 930-1 to 930-n. Spread modulators 912-1 to 912-n retrieve signal arrays from the corresponding data generators 911-1 to 911-n respectively and multiply spread codes (for example, Orthogonal Gold Codes or Walsh Codes) D1 to Dn. Here, different values are used for the respective spread codes D1 to Dn. Then, an adder 913 adds the spread modulated signal arrays M1 to Mn mutually so that a code division multiplexed signal array is generated.
FIG. 11 is a block diagram showing an example of an internal structure of the receivers 930-1 to 930-n. According to the receivers 930-1 to 930-n shown in FIG. 11, a matched filter 931 multiplies a code (one of the codes D1 to Dn) which is used for the coding on the code division multiplexed signal array to separate a signal array corresponding to the spread code. For example, in order to separate a signal array which is multiplied with the spread code D1, the spread code D1 is multiplied to the code divisional multiplexed signal array. A comparator 932 compares each signal bit value of the separated signal array with a threshold value. A signal array indicating a determination result is latched by a latch 933 and output being synchronized with a sampling clock. Accordingly, one of the signal arrays S1 to Sn (See FIG. 10) is reproduced.
As shown in FIGS. 9 to 11, the respective receivers 930-1 to 930-n use different spread codes D1 to Dn. In a typical communication system, the respective receivers 930-1 to 930-n stores a single spread code which is used for own code division demultiplexing and any other spread codes are not stored. Therefore, the respective receivers 930-1 to 930-n cannot restore signal arrays addressed to other receivers. Accordingly, security for users of the respective receivers 930-1 to 930-n is maintained.
In a typical communication system, the transceiver 910 does not always transmit data to all of the receivers 930-1 to 930-n. In other words, in many cases, one or more channels CH1 to CHn are unoccupied. Therefore, in such conventional communication system, there often exists a band which is not in use and so bandwidth efficiency is low.
Applying an unoccupied channel for a transmission addressed to another receiver may be viewed as a way to use the unoccupied band efficiently. However, in case that a single channel is shared by a plurality of receivers, the above described security cannot be maintained as a single spread code may be shared by the plurality of receivers. In addition, a plurality of matched filters 931 are required to be provided to the respective receivers 930-1 to 930-n, so the cost of the receivers will be increased.